In manufacture of a hot-rolled sheet steel, usually, a slab is charged into a heating furnace in an oxidizing atmosphere to be heated with a temperature within a range of 1100.degree.-1400.degree. C. extending over several hours. The heated slab is hot-rolled repeatedly by a rolling machine extending over a plurality of number of times so that a predetermined thickness thereof is obtained. A high temperature heating extending over several hours causes scale to be created on a surface of the slab. If the scale is subjected to a hot rolling process in such a state that the scale does not sufficiently break away, the scale will encroach on the surface of the slab and as a result remains as a scale defect. The scale defect on the surface of the slab remarkably damages a surface nature. In addition, the scale defect will become a starting point of cracks in a bending processing or the like. These will be a cause of serious damage to quality of products. In view of this matter, hitherto, there are proposed several ways to prevent an occurrence of scale defects on a slab surface (a sheet steel surface). As one of the ways, there is known a scheme in which a water jet descaling apparatus (hereinafter, referred to as a descaler) for ejecting water at a pressure, for example, about 100-150 kg/cm.sup.2 is disposed in a direction (a width direction of a sheet steel) which intersects substantially perpendicularly to a carrying direction of the sheet steel, and high pressure water is ejected from the descaler toward a surface of the sheet steel to separate and remove scale created on the surface of the sheet steel.
In general, according to the scheme as mentioned above, there are provided a plurality of arrays of the descaler each being equipped with a plurality of nozzles in a longitudinal direction thereof (a width direction of a sheet steel) to eject water toward the surface of the sheet steel. In order to prevent scale removed by water ejected from the respective nozzles from entering a rolling machine which is installed at the downward-stream end with respect to the carrying direction of the sheet steel, water is ejected from the descaler of each of the arrays toward the upward-stream end with respect to the carrying direction of the sheet steel. By the way, water ejected from the descaler disposed at the downward-stream end with respect to the carrying direction toward the upward-stream end with respect to the carrying direction flows on the surface of the sheet steel up to a collision area in which water ejected from the descaler disposed at the more upward-stream end with respect to the carrying direction than the noticed descaler collides with surface of the sheet steel. Hence, water ejected from the descaler disposed at the more upward-stream end with respect to the carrying direction than the noticed descaler does not collide directly with surface of the sheet steel, but collides once with water ejected from the descaler disposed at the more downward-stream end with respect to the carrying direction and flowing on the surface of the sheet steel. As a result, water ejected from the descaler disposed at the more downward-stream end with respect to the carrying direction serves as a cushion, so that an impact force of water ejected from the descaler disposed at the more upward-stream end with respect to the carrying direction to the surface of the sheet steel will be reduced. Thus, this will be a cause of such a problem that it is difficult to implement a sufficient descaling.
Further, as another method of the scale eliminating ways, there is proposed a method (refer to Japanese Patent Laid Open Gazette No. 502113/1984) in which as shown in FIG. 21, water 14a is ejected from a cooling header 14 disposed at the upward-stream end with respect to the carrying direction 12 of a sheet steel 10 toward the upward-stream end with respect to the carrying direction, while water 16a is ejected from a cooling header 16 disposed at the downward-stream end with respect to the carrying direction 12 toward the downward-stream end with respect to the carrying direction, and thus water 14a ejected from the cooling header 14 disposed at the upward-stream end flows on a surface of the sheet steel, as shown by arrow 14b, toward the upward-stream end with respect to the carrying direction, while water 16a ejected from the cooling header 16 disposed at the downward-stream end flows on the surface of the sheet steel, as shown by arrow 16b, toward the downward-stream end with respect to the carrying direction, whereby water ejected from the cooling header 14 and water ejected from the cooling header 16 do not interfere with each other on the surface of the sheet steel so as to collide directly with the surface of the sheet steel.
According to the method described in the Gazette referenced above, while water ejected from the cooling header 14 and water ejected from the cooling header 16 do not interfere with each other on the surface of the sheet steel, water ejected from each of a plurality of nozzles disposed on a single cooling header will be emitted with a spread. Hence, waters ejected from adjacent nozzles will interfere with each other on the surface of the sheet steel. A state of the interference of water on a surface of a sheet steel will be explained referring to FIG. 22. FIG. 22 is a typical illustration showing on a plan view basis the state of the interference.
To perform a descaling, there is a need to cause water to have a collision extending over overall width of the sheet steel 10 being transported in the carrying direction 12. Consequently, water is emitted from the respective nozzle in such a manner that collision areas 20 and 22, in which waters emitted from the adjacent nozzles disposed on a single descaler (not illustrated) collide with a sheet steel surface 10a, partially overlap. While it is desired that the overlapped area is as narrow as possible, usually, the nozzles are arranged in such a manner that the overlapped area having 5 mm-10 mm in a direction of a sheet steel width is formed, since spread of the collision areas 20 and 22 will be varied owing to a variation in a distance between the sheet steel 10 and the nozzles, which variation caused by a variation in thickness of the sheet steel 10, and a spread of the collision area differentiates owing to an error in manufacture of nozzles.
In the overlapping area, water-to-water ejected from the mutually adjacent nozzles collide with each other, so that the collision force is reduced. Consequently, it is difficult to sufficiently remove scale. In order to provide a narrower overlapping area, there is considered a scheme in which as shown in FIG. 23, collision areas 24 and 26 for waters ejected from the mutually adjacent nozzles are staggered with respect to the carrying direction 12, and waters are ejected from the respective nozzles toward the upward-stream end with respect to the carrying direction 12. However, since water ejected toward the upward-stream end with respect to the carrying direction 12 will be emitted with a spread, water in the collision area 24 will be spread on the sheet steel surface 10a toward the upward-stream end with respect to the carrying direction 12. Thus, a part of waters serves as a cushion for water ejected to the collision area 26. As a result, in the area shown by an arrow 28, it may be considered that water ejected from the nozzle does not collide directly with the sheet steel surface. Thus, there is a fear such that scale in this area can not be sufficiently removed.
In order to solve the problem as mentioned above, there is considered a scheme in which the respective nozzles are arranged at sufficient intervals with respect to the carrying direction, and before water ejected from a nozzle spreads up to a collision area in which said water will collide with water ejected from another nozzle, said water is removed from a sheet steel surface. However, this method involves undesired problems in operation, such that it is needed to provide a space for installation of nozzles arranged at sufficient intervals with respect to the carrying direction, and conditions of descaling or cooling conditions by descaling are different owing to variance in temperature conditions on the sheet steel surface with which waters ejected from the respective nozzles arranged at sufficient intervals with respect to the carrying direction collide.
By the way, the quality of separativeness of scale in removal of scale is largely affected by the operational conditions such as water pressure of a descaler, and in addition the nature of scale, that is, composition and structure of scale and the like. Specifically, it is known that a primary scale created on a steel, which is large in the Si (silicon) content, is very difficult to be separated. The reason why such scale is very difficult to be separated is that when the steel, which is large in the silicon content, is oxidized through high-temperature heating, Si contained in the steel is subjected to the selective oxidization to form 2FeO.SiO.sub.2 (fayalite) which is large in thermal plasticity, so that a sub-scale layer possessing such a characteristic structure that the interface with the steel is complicated is formed. A heat treatment of the steel containing, for example, Si not less than 0.1% increases remarkably an amount of the sub-scale mentioned above. This sub-scale cannot be easily removed, as mentioned above. Thus, an infinite number of scale defects remains on a surface of a product after a rolling process. This will be a cause of a remarkable degradation of commercial value of products. Further, it happens that the secondary scale, which will be formed after a removal of the primary scale, does not break away by the above-mentioned method of ejecting high pressure water. Hence, this is in danger of an occurrence of scale defects.
As a technique to solve the foregoing problem, Japanese Patent Publication No. 1085/1985 discloses "a descaling method at hot rolling for a steel containing Si in which when a slab consisting of a steel containing 0.10-4.00% of Si is subjected to a hot rolling process to produce a hot-rolled sheet steel, descaling by a high pressure water jet of 80-250 kg/cm.sup.2 is practiced not less than 0.04 seconds in a cumulative time during a rolling period of time in which a cumulative draft reckoning from a starting point of time of rolling is not less than 65% and an ingot piece temperature is 1000.degree. C. Further, Japanese Patent Laid Open Gazette No. 238620/1992 discloses "a descaling method in which when a difficult-separative scale of steel species is subjected to a hot rolling process to produce a hot-rolled sheet steel, a high pressure water spray, given by a collision pressure per unit spraying area between 20 g/mm.sup.2 and 40 g/mm.sup.2 and a flow rate between 0.1 liters/min.multidot.mm.sup.2 and 0.2 liters/min.multidot.mm.sup.2, is ejected on a surface of the sheet steel prior to a finishing rolling.
As a nozzle for separating and removing difficult-separative scale, Japanese Patent Laid Open Gazette No. 261426/1993 proposes "a descaling nozzle in which an rectifying liquid flow channel is arranged on a longitudinal basis". In this Gazette, it is disclosed that the use of the descaling nozzle having a rectifier may increase the collision force comparing with the conventional nozzle, and thus it is effective for the difficult-separative scale of steel species.
However, according to the technique disclosed in Japanese Patent Publication No. 1085/1985 among the above-mentioned prior arts, there is a need to ensure a high FET (Finisher Entry Temperature), such as 1000.degree. C. or more, and thus it is obliged to extract the sheet steel at high temperature. This involves such problems that unit requirement gets worse, and scale is increased. And in addition, the high temperature such as 1000.degree. C. or more causes various restrictions in draft and descale time. This will be a cause of a complicated work in rolling.
According to the technique disclosed in Japanese Patent Laid Open Gazette No. 238620/1992, the collision pressure and flow rate of the high pressure water spray are defined to separate scale by an instantaneous collision force. In this technique, it is considered that the separative amount of scale depends on the collision pressure of the high pressure water spray. This concept has been described in detail in a paper "Collision pressure at the time of high pressure water descaling in hot rolling" appearing in a publication "Iron and Steel", 77(1991), Vol. 9. This paper discloses that consideration of variations in thermal expansion caused by a quenching action for scale with high pressure water and the minimum collision pressure for separating scale created on the various kinds of steels permits descaling to be satisfactorily performed. However, according to the technique as mentioned above, while most of the scale components are separated, a scale component having such a structure that scale encroaches on a ground metal will not be removed and thus remains. Hence, even after rolling, the scale defect referred to as a red scale remains. There arises the problems that such a scale defect becomes remarkable as the Si content is increased.
The above-mentioned Japanese Patent Laid Open Gazette No. 261426/1993 discloses structure and performance of the descaling nozzle equipped with the rectifier, but fails to disclose a method of the use in a hot rolling factory, for instance, the optimum distance between the nozzle and the sheet steel surface.
As a method of removing scale created on a surface of sheet steel, there is disclosed a method in which a liquid is ejected from a nozzle with a supplying pressure between 1000 Kg/cm.sup.2 and 10000 Kg/cm.sup.2 so that droplets formed in a droplet stream area of the liquid collide with a surface of a sheet steel, thereby removing scale (refer to Japanese Patent Laid Open Gazette No. 138815/1992). However, according to the method referenced above, since the supplying pressure of the liquid is not less than 1000 Kg/cm.sup.2, there arises the problems that this method is unfavorable in economy and maintenance of facilities for supplying liquid.
In view of the foregoing, it is an object of the present invention to provide cleaning method and cleaning apparatus which may be preferably used, for example, when scale is removed from a surface of a sheet steel before a hot rolling process.